The electric power grid is a complex critical infrastructure network. Its inter-connectivity enables long-distance transmission of power for more efficient system operation. The same inter-connectivity, however, also allows the propagation of disturbances. In fact, blackouts due to cascading failures occur because of the intrinsic electrical properties of this propagation and physical mechanisms that are triggered by it. In this paper we propose a stochastic Markov model, whose transition probabilities are derived from a stochastic model for the flow redistribution, that can potentially capture the progression of cascading failures and its time span. We suggest a metric that should be monitored to expose the risk of failure and the time margin that is left to perform corrective action. Finally we experiment with the proposed stochastic model on the IEEE 300 bus system and provide numerical analysis.